1. Field of the Invention
The field of the present invention is tandem type vacuum boosters for boostingly operating a master cylinder for a brake of an automobile and the like by a vacuum pressure, and more particularly, is improvements of such tandem type vacuum boosters comprising a booster shell which is comprised of a front shell half formed into a bottomed cylindrical shape with a rear end thereof opened, a rear shell half which closes the opened end of the front shell half, and a partition wall plate clamped between both the shell halves for dividing a chamber between both the shell halves into a front shell chamber and a rear shell chamber; a front diaphragm having an outer peripheral bead clamped between the front shell half and the partition wall plate; and a rear diaphragm having an outer peripheral bead clamped between the partition wall plate and the rear shell half; the front shell half being provided at its rear end with an annular radially outwardly extending step and a cylindrical portion extending rearwardly from an outer periphery of the step, the partition wall plate and the rear shell half having a first and a second flange provided thereon respectively, which flanges are superposed one on another within the cylindrical portion and rotatable between a disassembled position and an assembled position, each of the flanges having a number of notches circumferentially arranged along a peripheral edge thereof, the flanges being connected to each other through positioning means so that their notches are aligned with each other, the cylindrical portion being provided with a number of locking claws which are aligned with the notches in the disassembled position of the flanges to enable engagement and disengagement of the flanges with and from the cylindrical portion and which clamp the flanges to the step in the assembled position.
2. Description of the Prior Art
Such tandem type vacuum boosters are already known, for example, as disclosed in Japanese Utility Model Publication No. 31330/87 and Japanese Utility Model Application Laid-open No. 157361/85.
The tandem type vacuum booster described in Japanese Utility Model Publication No. 31330/87 is designed so that the positioning means for connecting the flanges of the partition wall plate and the rear shell half also serves as stop means which adapted to abut against the locking claw upon rotation of the flanges to the assembled position to stop the flanges at the assembled position. With the booster of such a construction, the positioning means may be deformed in some cases by reception of a large load from the locking claw in the assembled position, and such deformation may bring about a misalignment in relative positional relation between both the flanges, resulting in an interference with the subsequent disassembling and reassembling. The tandem type vacuum booster described in Japanese Utility Model Application Laid-open No. 157361/85 is designed so that such the positioning means for the flanges of the partition wall plate and the rear shell half would not be interfered with the locking claws and may be thereby prevented from being deformed. With such booster, it is difficult to stop the flanges at an appropriate assembled position when the flanges have been rotated relative to the cylindrical portion of the front shell half during assembling.